Structure of silylated benzohydroxamic acids

NMR spectra (C-13, N-15 and Si-29) of the products of trimethylsilylation a nd tert-butyldimethylsilylation of benzohydroxamic acid and model derivativ es of benzohydroximic acid were studied in solutions. The products are show n to have the structure of (Z)(syn)-O,O'-bis(trimethylsilyl) and -O,O'-di(t ert-butyldimethylsilyl), derivatives of benzohydroximic acid [i.e. (Z)-trim ethylsilyl ester of N-(trimethylsiloxy)benzoimidic acid and (Z)-tert-butyld imethylsilyl eater of N-(tert-butyldimethylsilyl)benzoimidic acid, respecti vely]. Whereas N-15 NMR chemical shifts are the most useful NMR parameter f or differentiation between derivatives of hydroxamic and hydroximic tautome rs, differentiation between Z and E stereoisomers of hydroximic acid deriva tives is more reliable on the basis of (1)J(C-13, C-13) coupling within the C-C=N moiety. Copyright (C) 1999 John Wiley & Sons, Ltd.